(1) Field of the Invention
This invention relates to a rotary encoder correcting method for a transport apparatus which calculates transport speeds of a printing medium and timing of processes based on output signals of a rotary encoder connected to a transport roller rotatable synchronously with movement of the printing medium, and to a transport apparatus using this method.
(2) Description of the Related Art
A known method of this type is used with a transport apparatus which transports elongate roll paper. The apparatus includes a transport roller rotatable synchronously with movement of the roll paper, and a rotary encoder having a shaft thereof connected to the center of the transport roller. The method measures transport speed of the roll paper based on output signals of the rotary encoder (see Japanese Unexamined Patent Publication No. 2008-126516, for example). A printing apparatus having the above transport apparatus, for example, performs printing while adjusting timing of printing based on the transport speed calculated.
However, the conventional example with such construction has the following problem.
The conventional method sometimes fails to connect the shaft of the rotary encoder in precise alignment to the center of the rotary shaft of the transport roller. When the transport roller is misaligned in this way, there arises a problem of being unable to accurately determine timing such as transport speed based on the output signals of the rotary encoder.
That is, with the shaft of the rotary encoder attached accurately, instead of being misaligned, to the center of the rotary shaft of the transport roller, when the entire outer circumferential surface of the transport roller is divided at given predetermined angles about the shaft of the rotary encoder, the same length of the outer circumferential surface of the transport roller is obtained for all angular divisions. Thus, a length of the outer circumferential surface of the transport roller corresponding to a moving distance of the roll paper, and the number of output signals of the rotary encoder, are in a one-to-one relationship. It is therefore possible to calculate a transport speed of the roll paper accurately based on the number of the output signals of the rotary encoder and periods of time taken until the output signals are outputted. In the case of misalignment, on the other hand, since the length of the outer circumferential surface of the transport roller varies from angle to angle, a length of the outer circumferential surface of the transport roller corresponding to a moving distance of the roll paper and the number of output signals of the rotary encoder are not in a one-to-one relationship. It is therefore impossible to calculate a transport speed of the roll paper accurately based on the number of the output signals of the rotary encoder and periods of time taken until the output signals are outputted. That is, accurate timing cannot be calculated based on the output signals of the rotary encoder.